Top 6 SOLID Principles C# Interview Questions with code

The SOLID principles are a set of design guidelines that help developers create more understandable, flexible, and maintainable software.

These principles are essential for writing clean code and are widely discussed in technical interviews for C# developers.

In this blog, we'll explore the most commonly asked SOLID principles C# interview questions and provide clear, concise answers to help you prepare.

Page Contents

1. What are the SOLID principles?

2. Can you explain the Single Responsibility Principle (SRP)?

3. What is the Open/Closed Principle (OCP)?

4. Can you describe the Liskov Substitution Principle (LSP)?

5. What is the Interface Segregation Principle (ISP)?

6. Can you explain the Dependency Inversion Principle (DIP)?

1. What are the SOLID principles?

The SOLID principles are five design principles intended to make software designs more understandable, flexible, and maintainable. They are:

Single Responsibility Principle (SRP)
Open/Closed Principle (OCP)
Liskov Substitution Principle (LSP)
Interface Segregation Principle (ISP)
Dependency Inversion Principle (DIP)

2. Can you explain the Single Responsibility Principle (SRP)?

The Single Responsibility Principle states that a class should have only one reason to change, meaning it should have only one job or responsibility. This principle aims to reduce the complexity of code by ensuring that each class addresses a specific aspect of the program's functionality. For example:

public class Report
{
    public string ReportType { get; set; }

    public void GenerateReport()
    {
        /*Code to generate report*/
    }

    public void SaveReport()
    {
        /*Code to save report*/
    }
}

Here, Report class violates SRP as it handles both generating and saving the report. To follow SRP:

public class ReportGenerator
{
    public void GenerateReport()
    {
        /*Code to generate report*/
    }
}

public class ReportSaver
{
    public void SaveReport()
    {
        /*Code to save report*/
    }
}

3. What is the Open/Closed Principle (OCP)?

The Open/Closed Principle states that software entities (classes, modules, functions, etc.) should be open for extension but closed for modification. This means you should be able to add new functionality without changing existing code. For example:

public abstract class Shape
{
    public abstract double GetArea();
}

public class Circle : Shape
{
    public double Radius { get; set; }

    public override double GetArea() => Math.PI \* Radius \* Radius;
}

public class Rectangle : Shape
{
    public double Width { get; set; }

    public double Height { get; set; }

    public override double GetArea() => Width \* Height;
}

Here, you can add new shapes without modifying the existing code by extending the Shape class.

4. Can you describe the Liskov Substitution Principle (LSP)?

The Liskov Substitution Principle states that objects of a superclass should be replaceable with objects of a subclass without affecting the correctness of the program. This principle ensures that a subclass can stand in for its superclass. For example:

public class Bird
{
    public virtual void Fly()
    {
        /*Flying logic*/
    }
}

public class Eagle : Bird
{
    public override void Fly()
    {
        /*Flying logic specific to Eagle */
    }
}

public class Ostrich : Bird
{
    public override void Fly()
    {
        throw new NotImplementedException();
    }
}

Here, Ostrich violates LSP because it can't fly. A better approach:

public abstract class Bird { }

public class FlyingBird : Bird
{
    public virtual void Fly()
    {
        /*Flying logic */
    }
}

public class Eagle : FlyingBird
{
    public override void Fly()
    {
        /*Flying logic specific to Eagle */
    }
}

public class Ostrich : Bird
{
    /* Ostrich-specific logic */
}

5. What is the Interface Segregation Principle (ISP)?

The Interface Segregation Principle states that no client should be forced to depend on methods it does not use. Instead of one fat interface, many small, specific interfaces are preferred. For example:

public interface IWorker
{
    void Work();
    void Eat();
}

Here, the IWorker interface is too broad. To follow ISP:

public interface IWork
{
    void Work();
}

public interface IEat
{
    void Eat();
}

public class Worker : IWork, IEat
{
    public void Work()
    {
        /*Work logic */
    }

    public void Eat()
    {
        /*Eat logic */
    }
}

Now, classes can implement only the interfaces they need.

6. Can you explain the Dependency Inversion Principle (DIP)?

The Dependency Inversion Principle states that high-level modules should not depend on low-level modules. Both should depend on abstractions (e.g., interfaces or abstract classes). Additionally, abstractions should not depend on details. Details should depend on abstractions. For example:

public class LightBulb
{
    public void TurnOn()
    {
        /*Turn on logic */
    }

    public void TurnOff()
    {
        /*Turn off logic */
    }
}

public class Switch
{
    private LightBulb _bulb;

    public Switch(LightBulb bulb)
    {
        _bulb = bulb;
    }

    public void Operate()
    {
        /*Switch logic */
    }

}

Here, Switch depends on the LightBulb class, which violates DIP. To follow DIP:

public interface ILightBulb
{
    void TurnOn();
    void TurnOff();
}

public class LightBulb : ILightBulb
{
    public void TurnOn()
    {
        /*Turn on logic */
    }

    public void TurnOff()
    {
        /*Turn off logic */
    }
}

public class Switch
{
    private ILightBulb _bulb;

    public Switch(ILightBulb bulb)
    {
        _bulb = bulb;
    }

    public void Operate()
    {
        /*Switch logic */
    }

}

Now, Switch depends on the ILightBulb interface, not the concrete LightBulb class.

Conclusion

Understanding the SOLID principles is essential for writing clean, maintainable, and scalable code in C#.

These principles help ensure that your code is robust and can adapt to changes over time.

By familiarizing yourself with these commonly asked questions and their answers, you'll be well-prepared to discuss the SOLID principles in your next C# interview. Good luck!

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